Different sizes for /var/lib/docker - linux

Question

I don't know actually if this is more a "classic" linux or a docker question but:
On an VM where some of my docker containers are running I've a strange thing. /var/lib/docker is an own partitionwith 20GB. When I look over the partition with df -h I see this:
eti-gwl1v-dockerapp1 root# df -h
Filesystem Size Used Avail Use% Mounted on
devtmpfs 7.8G 0 7.8G 0% /dev
tmpfs 7.8G 0 7.8G 0% /dev/shm
tmpfs 7.8G 815M 7.0G 11% /run
tmpfs 7.8G 0 7.8G 0% /sys/fs/cgroup
/dev/sda2 12G 3.2G 8.0G 29% /
/dev/sda7 3.9G 17M 3.7G 1% /tmp
/dev/sda5 7.8G 6.8G 649M 92% /var
/dev/sdb2 20G 47M 19G 1% /usr2
/dev/sdb1 20G 2.9G 16G 16% /var/lib/docker
So usage is at 16%. But when I now navigate to /var/lib and do a du -sch docker I see this:
eti-gwl1v-dockerapp1 root# cd /var/lib
eti-gwl1v-dockerapp1 root# du -sch docker
19G docker
19G total
eti-gwl1v-dockerapp1 root#
So same directory/partition but two sizes? How is that going?

Answer 1

This is really a question for unix.stackexchange.com, but there is filesystem overhead that makes the partition larger than the total size of the individual files within it.

Answer 2

du and df show you two different metrics:
du shows you the (estimated) file space usage, i.e. the sum of all file sizes
df shows you the disk space usage, i.e. how much space on the disk is actually used
These are distinct values and can often diverge:
disk usage may be bigger than the mere sum of file sizes due to additional meta data: e.g. the disk usage of 1000 empty files (file size = 0) is >0 since their file names and permissions need to be stored
the space used by one or multiple files may be smaller than their reported file size due to:
holes in the file - block consisting of only null bytes are not actually written to disk, see sparse files
automatic file system compression
deduplication through hard links or copy-on-write
Since docker uses the image layers as a means of deduplication the latter is most probably the cause of your observation - i.e. the sum of the files is much bigger because most of them are shared/deduplicated through hard links.

Answer 3

du estimates filesystem usage through summing the size of all files in it. This does not deal well with the usage of overlay2: there will be many directories which contain the same files as contained in another, but overlaid with additional layers using overlay2. As such, du will show a very inflated number.
I have not tested this since my Docker daemon is not using overlay2, but using du -x to avoid going into overlays could give the right amount. However, this wouldn't work for other Docker drivers, like btrfs, for example.